Aluminum phosphate gels and precipitates are old in the art. U.S. Pat. No. 3,342,750 to K. K. Kearby indicates that aluminum phosphate is described in the prior art for use as a catalyst, and Kearby's contribution is the discovery of what he describes as a true hydrogel of aluminum phosphate which has a high surface area and which is thermally stable. Kearby teaches in Column 1, lines 62-66, that the prior art aluminum phosphate compounds were precipitates rather than hydrogels and that they have relatively low surface areas and poor heat stability. In Example 2 in Column 3 of Kearby, he teaches that the conventional method of making aluminum phosphate involves neutralizing an aqueous solution of aluminum chloride and phosphoric acid by slowly raising the pH of the acidified aluminum cation containing solution by the addition of base until a pH of about 7 is achieved. A more recent U.S. Pat. No. 3,904,550 to Lloyd A. Pine, entitled "Hydrocarbon Conversion Catalyst Comprising Alumina and Aluminum Phosphate", discusses Kearby's hydrogels and indicates the Kearby's precipitates are sensitive to water and difficult to form into a catalyst shape since they lose a substantial part of their surface area during the forming process (Col. 1, lines 26 et seq.). Both the Kearby and Pine references are assigned to the same assignee. Pine indicates that he has overcome the difficulties of the Kearby aluminum phosphate materials by preparing his aluminum phosphates as precipitates from an aluminum alkoxide. Such aluminum alkoxides are pyrophoric in nature and are thereby much more difficult to handle and are considerably more expensive than the more common aluminum inorganic salts such as aluminum chloride and aluminum nitrate.
The procedure of Kearby for producing a true aluminum phosphate hydrogel and the procedure of Pine to produce an alumina-aluminum phosphate precipitate which does not have the water sensitivity of the Kearby hydrogels are both quite complicated and thus expensive from a commercial standpoint. There is thus a need for a simplified method of preparing an alumina-aluminum phosphate composite precipitate by a simple technique whereby the precipitates are usable in catalytic applications by possessing sufficient thermal stability. In addition it is desirable from a catalytic application standpoint that such alumina-aluminum phosphate composition precipitates have as large an average pore radius as possible to admit the larger size molecules to the internal portions of the support where reaction can occur and subsequently to allow the molecules which are formed an easy access out of the catalyst support material.
A new class of thermally stable composite precipitates containing from 10 to 60 mole percent alumina and from 40 to 90 mole percent aluminum phosphate have now been discovered which have surface areas after calcining of from about 100 to about 200 m.sup.2 /g and which have an average pore radius of from 75 to 150 A. Calcining occurs at temperatures from 300.degree. to 500.degree. C. for times up to 16 hours. That such materials are thermally stable is evidenced by the fact that these materials after additional calcination at temperatures up to 900.degree. C. and times of up to 16 hours lose no more than about 30% of the surface areas which is available at a calcination temperature of 500.degree. C. for 16 hours. As will be shown below, the thermal stability of the materials of this invention is surprising in view of their high average pore radii.
The above new class of thermally stable alumina-aluminum phosphate composite precipitates are prepared by:
forming a first aqueous solution of aluminum cations and PO.sub.4.sup.--- anions wherein the molar ratio of the PO.sub.4.sup.--- anions to the aluminum cations is 0.82:1 to 0.25:1;
neutralizing said first aqueous solution with ammonia gas or a second aqueous solution of an ammonia based material selected from the group consisting of ammonium hydroxide, ammonium carbonate, ammonium bicarbonate or urea in such a manner that the pH of the combined first aqueous solution and the neutralizing medium is controlled in the range of 7 to 10;
recovering said precipitate;
drying said precipitate; and
calcining said precipitate at a temperature from 300.degree. to 500.degree. C. for a time up to 16 hours to form an alumina-aluminum phosphate composite material having a surface area from about 100 to about 200 m.sup.2 /g, an average pore radius from 75 to 150 A, and wherein no more than a 30% decrease in surface area is obtained if said composite precipitate is calcined at 500.degree. C. for 16 hours and is further calcined at a temperature up to about 900.degree. C. for up to 16 hours.